(1) Technical Field
The present invention relates to materials that respond to external stimuli either actively by changing their shape or passively by changing their mechanical properties. More specifically, the present invention relates to such “active materials” (materials that change shape in response to stimuli) and “passively shapeable materials” (materials that are controllably malleable) that are used in combination to permit the re-shaping of a device into at least one desired shape.
(2) Description of Related Art
The field of smart materials and intelligent structures has been gradually developing over the past few decades, increasingly enabled by technological advances in the areas of sensors, engineering materials, and actuators. The basis of many actuator and sensor technologies has increasingly been found in emerging “active materials.” Active materials, as a category, are materials that change their shape in response to an external control stimulus, typically a field, such as a thermal, magnetic, or electric field, but also radiation (light) or a changing chemical environment. Materials in this broad category include several classes, often delineated by the stimulus and material type: shape memory alloys, shape memory polymers, piezoelectric ceramics, magnetostrictives, and electroactive polymers. Within each of these classes, there are many materials; e.g., within electroactive polymers alone there are a wide variety of low- and high-voltage activated materials with widely varying properties, such as ionic-polymer metal composites, conductive polymers, gels, and others.
One class of active materials, those with shape memory, has been studied since the 1950s. The broad class of materials with shape memory includes materials such as shape memory polymers (SMPs) and shape memory alloys (SMAs). In particular, nitinol and other alloys that possess memory properties have been used in applications from catheter wires to eyeglasses. Discovered more recently, SMPs, on the other hand, have distinct advantages over SMAs. Typically, changes in chemical formulation enable a wider range of transformation temperatures and are more suitable to applications such as in surgical instruments. To-date, most applications of SMPs and SMAs have been distinct, for example, they have not been applied in a manner that would take advantage of the beneficial properties of both in a synergistic manner.
Often, for the purpose of controlling the fixed shape of a structure, active materials have been paired with or used in a composite structure with passive engineering materials to form an “active structure.” Upon activation, the overall shape of the structure becomes defined by a balance between the forces developed by the active material and the elastic energy stored in the passive engineering material structure.
An example of the state of the art in the patent literature which uses this principle is presented in U.S. Patent Publication No. 20010006207 by Caton et al., wherein the applicants describe the concept of SMA ribs in a passive elastomeric matrix for aircraft control surfaces. In the device described by Caton et al., elastomeric panels are moved between two positions by force exerted by a set of actuating ribs. In this case, power must be continuously applied to the SMA ribs to maintain the structure in the activated state.
In another example of related art, US Application 20020142119 “Shape memory alloy/Shape memory polymer tools” teaches the use of composite shape memory alloy (SMA), shape memory polymer (SMP) and combinations of SMAs and SMPs to produce catheter distal tips, actuators, etc., which are bistable. In this application, the SMA material is stabilized by the SMP material, and power-off hold can be accomplished using the SMP material. However, within the scope of the invention, the structure is generally limited in conception to structures with two states.
Other current literature in the art includes UK Pat. No. GB2280957, titled “A Surface Device Configurable by Shape Memory Actuation,” by Taylor et al. In the description by Taylor et al., individual sections of a surface are actuated between two positions using the one-way SMA effect to form a reconfigurable surface. Fluid pressure or springs are used to return a surface and a set of SMA tendons to their original position. In order to maintain the surface shape, the SMA wires must be continuously powered. Such controllable structures based on SMA materials suffer from a major drawback: while SMA materials are capable of high force actuation, the strength of the SMA material reduces considerably in its ‘power off’ state.
In many cases, power must be continuously applied to the structure, and such a requirement may limit the applicability of a particular “active structure” approach. It is possible to utilize the high and low temperature forms of an SMA. An example of a device that incorporates this concept and that uses a combination of, parts made of SMAs (coupled with a passive truss structure) was developed by the group at University of Virginia, led by Prof. Hayden Wadley, in the area of actively controlled cantilevers. A presentation from the Wadley group was made at the 9th Annual International Symposium on Smart Structures and Materials 17-21 Mar. 2002, San Diego, Calif., entitled “Shape-memory based structural actuator panels” by D. M. Elzey, A. Y. N. Sofia, H. N. G. Wadley, University of Virginia. In this presentation, a structure was described that consisted of a pair of SMA panels connected by a truss structure to form a cantilever. This structure could be actuated, enabling the position of the end of the cantilever to be controlled. With power off, the structure would then hold its shape. The major disadvantage of this structure is that the zero-power hold feature depends on the mechanical properties of the SMA when it is cold, which can be very poor.
While the above description is limited to SMA materials, similar examples and arguments can be made regarding active structures that utilize piezoelectric ceramics and polymers, magnetostrictive materials, electroactive polymers (such as dielectric elastomers, ionic polymer-metal composites, or ionic polymer gels), or liquid crystal elastomers. In certain cases, it is possible to utilize bi-stable structures to enable a structure to be transformed by the active material from one shape to another shape, but such bi-stable structures can be limited in that typically only two basic shapes can be held with reasonable stability; additionally bi-stable structures are limited in that sufficient external forces can overcome the bi-stability and cause undesirable switching between the two shapes.
In light of the above, there exists a need in the art for a material system that can achieve a broad range of shapes and that provides a reconfigurable surface that holds its shape without power. It is further desirable that the element system be capable of self-resetting via antagonistic action.